FIG. 3 is a block diagram showing an example of a conventional video data transfer system. As shown in the figure, video data decoded by a video decoder 11 is sent via a video port 12 to a graphic accelerator 30 having a video input function. Upon receiving video data, the graphic accelerator 30, which comprises a video processor 21, display control circuit 22, and a FIFO memory 24, causes the video processor 21 to perform predetermined signal processing for the received video data and outputs the processed video data to a frame buffer 14 via a frame buffer data bus 13 for storage in an internal off-screen memory 15.
Video data stored in the off-screen memory 15 is usually sent to the display control circuit 22 in the graphic accelerator 30 via the frame buffer data bus 13, converted to signals suitable for display, and then sent to a display 16. The frame buffer data bus 13, the frame buffer 14, and the off-screen memory 15 constitute a real time output path 25.
At the same time, video data sent from the off-screen memory 15 via the frame buffer data bus 13 is stored temporarily in the FIFO memory 24 which is in the graphic accelerator 30 and which constitutes a path 27 via which video data is output at video capture time. The video data is then output to a system bus 17 of a computer or some other unit and sent to a system memory 18.
However, there are certain problems encountered in the course of the investigations towards the present invention. There is a problem with the conventional system described above that, because the frame buffer data bus 13 is occupied by video data to be displayed on the display 16, the frame buffer data bus 13 becomes a bottleneck of data capturing, thus reducing the capturing rate.
Conversely, while video capturing is executed, the frame buffer data bus 13 of the conventional video data transfer system described above is occupied by video data transferred to the system memory 18. This prevents video data in the off-screen memory 15 from being updated and decreases the amount of updated data to be sent to the display 16 during data capturing, thus reducing the video display rate.